libdpkg: Add missing <stddef.h> to dpkg/string.h needed for size_t and NULL

This makes the header stand alone.

lib/dpkg/string.h

@@ -21,6 +21,7 @@
 #ifndef LIBDPKG_STRING_H
 #define LIBDPKG_STRING_H
 
+#include <stddef.h>
 #include <stdbool.h>
 
 #include <dpkg/macros.h>